This invention relates generally to mass transfer columns and, more particularly, to an apparatus and method for feeding a vapor or a vapor and liquid mixed phase stream into the column in a manner that allows a more uniform distribution of the vapor or mixed phase stream within the column. As used herein, the term “mixed phase stream” is intended to refer to a stream containing both vapor and liquid.
Mass transfer columns, including heat exchange columns, typically include an upright shell and a plurality of zones within the shell where contacting devices such as random or structured packing and/or horizontally disposed trays are used to facilitate mass or heat transfer between liquid streams flowing within the column. The liquid streams are normally decending liquid streams and ascending vapor stream, but other combinations of fluid streams are possible. As the liquid streams spread over the surface of the packing or tray, the surface area of contact between the streams increases, thereby facilitating the desired mass or heat transfer between the fluid streams. In order to increase the efficiency of the mass or heat transfer that takes place between the fluid streams, it is important that the fluid streams be uniformly distributed across the horizontal cross section of the column. When the fluid streams are vapor and liquid streams flowing countercurrently, this need for uniform distribution is particularly important at the lower vapor-liquid interface where ascending vapor enters the packing or other contacting device.
In columns of the type described above, vapor or mixed-phase streams are often introduced radially or tangentially into the column through a feed nozzle located below the zone containing the packing or other contacting device. The deflecting surfaces also facilitate de-entrapment of liquid droplets from mixed phase streams as the liquid droplets splash against and then flow downwardly along the deflecting surfaces. The vapor stream then rises through the zone and interacts with liquid flowing downwardly through the zone. Various feed devices, commonly referred to as vapor horns, have been developed in an attempt to interrupt the radial or tangential flow of the vapor or mixed phase stream as it enters the column from the feed nozzle and redirect it so that it rises in a more uniformly distributed manner across the horizontal cross section of the contact zone. These feed devices commonly utilize a generally annular passageway formed by the spacing between the column shell and an inner wall spaced radially inwardly from the shell. A top plate normally closes the top of the passageway and the vapor or mixed phase stream exits the passageway through the open bottom and/or through openings provided in the inner wall of the feed device. A deflector having oppositely directed deflecting surfaces is normally position at the location where the vapor or mixed phase stream enters the annular passageway. The deflecting surfaces of the deflector cause the vapor or mixed phase stream to be split into two equal or unequal streams that flow in opposite circumferential directions in the annular passageway. In the feed device disclosed in U.S. Pat. No. 5,106,544 to Lee et al., internal vanes are also positioned within the passageway and are oriented to redirect the vapor or mixed phase stream downwardly through the open bottom of the feed device.
The vapor or mixed phase stream entering the feed device from the nozzle can exert a large and/or unbalanced force on the deflecting surface of the deflector and the inner wall of the feed device. For example, when the feed nozzle is large and carries a high-speed vapor or mixed phase stream, the deflector and inner wall must have sufficient mechanical strength to withstand the large forces exerted by the vapor or mixed phase stream. In other situations, the vapor or mixed phase stream is not evenly split by the deflector and greater forces are exerted on the side of the deflector carrying the greater portion of the vapor or mixed phase stream. While the deflector and inner wall can be constructed of thicker gauge material to withstand these large and unbalanced forces, the additional weight of the thicker gauge material is generally undesirable because the component pieces of the feed device are normally hand carried into and assembled within the column.
In another approach that has been utilized to reduce the large, unbalanced force exerted by the vapor or mixed phase stream on the deflector and inner wall of the above-described feed devices, one or more upright vanes are used to subdivide the passageway on one side of the deflector into multiple subpassages and thereby reduce the volume of vapor flow that impacts the deflector and inner wall. The vanes are positioned in lateral spaced relationship at the feed device inlet where the vapor or mixed phase stream exits from the feed nozzle and enters the annular passageway. The vanes present an outer edge to the vapor or mixed phase stream exiting from the nozzle and are curved along their horizontal length to turn the vapor or mixed phase stream from its entry flow direction to a circumferential flow direction. Previously, the vanes have been used only on one side of the deflector to absorb large, unbalanced forces exerted by the vapor or mixed phase stream entering the feed devices. Use of these vanes has been limited to columns in which the nozzle diameter is large, i.e., on the order of 5 feet or greater, and unbalanced forces are also present because the vapor or mixed phase stream is split into two unequal portions by the deflector. The use of the vanes with smaller nozzles as well as those having equal flow splits has been thought to be unnecessary because the force of the vapor or mixed phase stream is small enough and balanced on the deflecting surfaces of the deflector so that normal gauge materials can readily absorb the impact of the entire vapor or mixed phase stream. Moreover, the use of vanes other than for force reduction purposes has been thought to be undesirable because of the expected increase in pressure drop resulting from frictional losses as the vapor or mixed phase stream flows along the surface of the vanes. As a result, it is believed that vanes of the type described have not previously been used with radial nozzles having equal vapor flow splits.